Scaler network analysis is a method of measuring the magnitude of transmission and impedance of a circuit as a function of the frequency of an input signal. Scaler network analyzers are devices for measuring impedance and transmission functions of linear networks at specific frequencies or over predetermined frequency ranges. There are two primary types of scaler network analyzers: AC detection and DC detection.
In DC detection network analysis, an unmodulated or CW signal is supplied to a device under test. An output CW signal from the device under test is supplied to a DC network analyzer. The CW signal is rectified by the DC network analyzer to produce a DC signal with a magnitude related to the amplitude of the CW signal supplied to the DC network analyzer. The DC network analyzer displays the magnitude of the DC signal as a function of the frequency of the corresponding CW signal.
In AC detection network analysis, a modulated or pulsed RF signal is supplied to a device under test. The pulsed RF signal is output from the device under test and supplied to an AC network analyzer. The pulsed signal is rectified by the AC network analyzer to produce an AC signal modulated at a predetermined frequency. The AC signal has an amplitude related to the amplitude of the pulsed RF signal supplied to the AC network analyzer. The AC network analyzer displays the amplitude of the AC signal as a function of the frequency of the corresponding pulsed RF signal.
The major disadvantage of AC detection network analysis is that some devices do not respond properly to a pulsed RF signal Some examples of these devices are narrow bandwidth filters, active devices that are power sensitive, and some feedback loop systems. In these situations, the signal supplied to the device under test must be a CW signal, thus requiring DC detection. There are several network analysis configurations used when the signal supplied to the device under test must be a CW signal, however none have been completely satisfactory. One of the configurations utilizes an external modulator placed between the device under test and the AC network analyzer, while another configuration uses a DC detection network analyzer.
The use of a modulator placed between the device under test and the AC detection network analyzer has some major drawbacks. A significant error could be introduced as a result of the modulator's attenuation and SWR variation.
The alternative solution of using a DC network analyzer has several drawbacks of its own. DC network analyzers have a limited dynamic range due to broadband noise from the signal source. Typical broadband sweepers limit the low sensitivity measurement to about -40 dBm due to this noise. Additionally, drift associated with DC coupled amplifiers will introduce a significant error when measuring input signals near or below -50dBm.